Energy density - High-field analyzes (HFA)

7. High-field analyzes (HFA)

7.1 Energy density

The energy density of the samples is calculated from the HFA measurements and indicates the dominant magnetic property in the rock. In Fig. 7.1 the logarithmic ratio of the energy density of paramagnetic and ferrimagnetic minerals is plotted versus the bulk susceptibility of the samples. The resulting curve shows a correlation between the dominant magnetic property versus the bulk susceptibility obtained from the AMS measurements. From Fig. 7.1 it is obvious that the higher the bulk susceptibility, the more pronounced is the domination of ferrimagnetic minerals. Samples with bulk susceptibilities below 1000*10^-6 SI usually have ratios of para- over ferrimagnetic energy densities higher than 1 although some exceptions do occur. Samples with bulk susceptibilities >1000*10^-6 SI are controlled by the ferrimagnetic minerals and show ratios lower than 1. With higher bulk susceptibilities the ratio seems to vary within the range of 0.1 – 0.3.

Fig. 7.1: Energy density (log) versus bulk susceptibility of the HFA samples. Except for two samples a strong trend is obvious with an increasing domination of ferrimagnetic minerals with increasing bulk susceptibility. Samples from the different lithological units can not be differentiated according to their amount of ferrimagnetic minerals.

0 1000 2000 3000 4000 5000 6000 7000 8000

k_bulk*10^-6 SI

ener. dens. (para) / energ. dens (ferri)

SGG NGG WGG EQG PG

7.2 Southern gneissic granites

For the southern gneissic granites, samples 42a, x1 and 128bII were chosen for the measurements. The samples show bulk susceptibilities of 530-, 595- and 1670*10^-6 SI units, respectively. The AMS measurements revealed a weakly oblate magnetic ellipsoid (T=0.15; U=0.117) and a high degree of anisotropy (P’ = 1.144) for sample 42a, a moderately oblate magnetic ellipsoid (T=0.409; U=0.38) and a high degree of anisotropy (P’ = 1.148) for sample x1 and a weakly oblate magnetic ellipsoid (T=0.274; U=0.258) and a low degree of anisotropy (P’ = 1.072) for sample 128bII (see Fig. 7.2). The orientation of the main magnetic axes from the different magnetic measurements is shown in Fig. 7.3.

Fig. 7.2: Comparison of the form-parameters U of the different measurements (AMS and HFA).

The different samples show a different behavior concerning the interaction of paramagnetic and ferrimagnetic properties.

Fig. 7.3: Comparison of the orientation of the different axes of the magnetic ellipsoid from different measurements of magnetic properties. The orientation of the ferrimagnetic axes usually shows a perfect correlation with the axes of the AMS measurement indicating a domination of the ferrimagnetic properties. The orientation of the paramagnetic axes seems to be rotated with respect to the AMS measurement.

42a 128b

Para

Ferri

AMS X1

N N N

k₃ k1

-1 -0.5 0 0.5 1

42a x1 128bII

Para Ferri AMS

In sample 42a the magnetic properties of the rocks seems to be controlled by the ferrimagnetic properties since the shape of the magnetic ellipsoid of the AMS measurements and the ferrimagnetic properties exhibit the same values (see Fig.

7.2). In sample x1 the different properties seem to be interacting slightly destructive while in sample 128bII the interaction of the magnetic properties seems to be constructive (see Fig. 7.2). This observation, however, is not confirmed by the orientation of the main axes of the respective magnetic ellipsoids (see Fig. 7.3), where a destructive orientation of the paramagnetic axes to the nearly perfectly matching AMS and ferrimagnetic axes can be observed. In sample 42a the axes of the ferrimagnetic ellipsoid perfectly match those of the AMS ellipsoid while the axes of the paramagnetic ellipsoid are rotated. In sample x1 small differences in the orientation of the ferrimagnetic and AMS axes and the rotation of paramagnetic axes contribute to the bulk magnetic properties of the rock sample. In samples 42a and 128bII the ferrimagnetic fabric is much better defined than the paramagnetic fabric. In sample x1 the paramagnetic fabric is much better defined than the ferrimagnetic fabric (see Fig. 7.4).

Fig. 7.4: Diagram of the l, f and p parameters of the respective samples. While in sample 42a the ferrimagnetic fabric is much better developed than the paramagnetic fabric, for sample 128bII only a slightly more pronounced ferrimagnetic than paramagnetic fabric was calculated. In sample x1 the paramagnetic fabric is better developed than the ferrimagnetic fabric. However, the samples do show a pronounced planar fabric which is confirmed by the measured form of the respective ellipsoids.

This is confirmed by the pronounced oblate shape of the paramagnetic ellipsoid and the accompanying high value of P’. Furthermore, f (see equation [10], chapter 5.4.2) reflects the oblate magnetic ellipsoid as well while l (see equation [9], chapter 5.4.2) is smaller by a factor of ~2.6 (f=6.355, l=2.46, see Fig. 7.4).

7.3 Northern gneissic granites

Seven samples have been chosen from the northern gneissic granites ranging in their bulk susceptibilities from 500 to 4600*10^-6 SI. While the AMS measurements

0.01 0.1 1 10 100 1000

42a 128bii x1

l f p dominating paramagentic

dominating ferrimagnetic

revealed uniformly high T values for most of the samples (T=0.3 – 0.8), sample 63d (T=0.051) and 48c (T=-0.073) show neutral or nearly neutral values (see Fig.

7.5). The orientation of the respective magnetic axes exhibits a parallelism of the axes for most of the samples, only in samples 56aII and 44a there is no perfect alignment of the respective magnetic axes (see Fig. 7.6). In sample 63d the paramagnetic axes are slightly shifted with respect to the other axes.

Fig. 7.5: Comparison of the form-parameter U of the northern gneissic granites.

The ferrimagnetic and AMS-axes of sample 63d show a good concordance in k1

and k2, only k3 of the measurement shows small differences in its orientation.

Since the axes of the same measurements have to be perpendicular with respect to each other this probably reflects a measurement error. In samples 56aII and 44a the ferrimagnetic and AMS axes are perfectly aligned, the paramagnetic axes k1 and k2 seem to have interchanged in comparison to the ferrimagnetic or AMS axes.

Fig. 7.6: Comparison of the orientation of the different main magnetic axes of the different measurement methods. It is confirmed by the respective orientations that in most of the measured samples the ferrimagnetic minerals do not necessarily dominate the orientation of the magnetic ellipsoid. For example sample 53a points to a contribution of both, paramagnetic and ferrimagnetic properties to the orientation of the AMS ellipsoid.

44a 56aII 73a 63d 57a 53a 48c

Para Ferri AMS

Sample 53a shows a domination of paramagnetic properties over the bulk magnetic properties of the sample (see Fig. 7.5). This is also indicated by the orientation of the AMS axes between the respective paramagnetic and ferrimagnetic axes. Furthermore, the magnetic properties show a more distinct paramagnetic fabric than ferrimagnetic fabric (l=3.144, f=7.517, see Fig. 7.7). This is confirmed by the highly oblate form of the paramagnetic ellipsoid (Upara=0.59, Uferri=0.03). The same holds true for sample 48c, still, as can be deduced from the shape of the respective magnetic ellipsoids, the ferrimagnetic properties dominate the paramagnetic properties.

Fig. 7.7: l, f and p parameters of the samples. In most samples the ferrimagnetic anisotropies are better developed, only sample 53a shows a more distinct paramagnetic fabric. Samples 57a and 48c show nearly equally developed fabric anisotropies.

7.4 Western gneissic granites

The six selected samples for the western gneissic granites (140aI, 133d, 116bI, 108bII, 125aII and 123b) show oblate (140aI, 116bI and 125aII) as well as prolate (133d, 108bII and 123b) magnetic ellipsoids (see Fig. 7.8).

Fig. 7.8: Comparison of the U-parameters for the western gneissic granites. Again only a weak correlation between the AMS measurements and the paramagnetic properties of the HFA measurements is obvious pointing to a domination of the ferrimagnetic properties over the paramagnetic properties for most of the samples.

0.01

73a 63d 57a 53a 48c 56aii 44a

The distinctness of the ellipsoids is moderate with U-values in the range of –0.4 and 0.4. The orientations of the respective axes of the different measurements do not show a pronounced concordance of axes, but rather seem to be only loosely connected to each other (see Fig. 7.9). Sample 116bI shows a marked alignment of the paramagnetic axes with the AMS axes indicating a domination of paramagnetic properties of the sample. The other samples either show a correlation between the ferrimagnetic axes and the AMS axes (125aII, 133d and 108b) or the axes are rotated or interchanged (140aI, 125a and 123b).

Fig. 7.9: The correlation of the orientation of the magnetic axes for most of the samples is weak. A mixture of ferrimagnetic and paramagnetic properties is responsible for the orientation of the AMS ellipsoid. Only sample 108bII shows a pronounced domination of the ferrimagnetic properties over the paramagnetic properties indicated by the perfect alignment of the respective magnetic axes.

The ratios of l and f reflect the above stated observations (see Fig. 7.10). For sample 116bI unusually high values were calculated (l=124, f= 87, see Fig. 7.10).

Together with the low bulk susceptibility and the orientation of the respective magnetic axes only very small amounts of ferrimagnetic minerals seem to be present in the sample. This is confirmed by the low degree of bulk susceptibility (133*10^-6 SI) . The other samples of the western gneissic granites with their destructive and constructive alignment of magnetic axes are characterized by a more distinct fabric of the ferrimagnetic minerals present than that of the paramagnetic minerals.

116b 140aI 133d

108b 125a 123b

k₂ k3

Para

Ferri

AMS

N N N N N N

k₂ k₃

Fig. 7.10: l, f and p parameters of the samples. Only in sample 116bI the paramagnetic anisotropy is more pronounced than the ferrimagnetic anisotropy.

7.5 Equigranular granite

Only two samples were used from the granitoids of the equigranular granites (146aII and 136c). Sample 136c has a bulk susceptibility of 133*10^-6 SI while sample 146aII has a bulk susceptibility of ~2000*10^-6 SI. For sample 136c, the low susceptibility points to a control of the paramagnetic properties over the bulk rock properties. The magnetic ellipsoid shows a neutral shape of the paramagnetic ellipsoid and a pronounced prolate shape of the ferrimagnetic ellipsoid. However, the AMS measurements revealed a moderately oblate magnetic ellipsoid (see Fig.

7.11).

Fig. 7.11: U-parameters for the samples of the equigranular granites.

The orientation of the different axes show a concordance between the paramagnetic and the ferrimagnetic axes for sample 136c, still the AMS axes are rotated (see Fig. 7.12). This might reflect the occurrence of other ore minerals in the sample whose magnetic properties control the orientation of the AMS that can not be measured during the separation process due to its ferromagnetic or antiferromagnetic behavior. This is confirmed by the contrasting values of the respective shapes of the magnetic ellipsoids. Sample 146aII again shows a good concordance between the ferrimagnetic and AMS axes (see Fig. 7.12), and a

0.01 0.1 1 10 100 1000

140ai 133d 116bi 123b 125aii 108bii

l f p dominating paramagentic

dominating ferrimagnetic

-1 -0.5 0 0.5 1

146aII 136c

Para Ferri AMS

constructive connection of the paramagnetic and ferrimagnetic properties (see Fig.

7.11).

The magnetic fabric of sample 146aII has l- and f-values of ~1 (see Fig. 7.13). The shape of the magnetic ellipsoid from the AMS measurements suggests a domination of the ferrimagnetic over the paramagnetic properties of the sample.

Sample 136c shows equally evolved planar fabrics (f=0.868), the linear fabric of the ferrimagnetic minerals seems to be far better defined (see Fig. 7.13) which is reflected in the shape-parameters (Upara=-0.02, Uferri=-0.87).

Fig. 7.12: Orientations of the main magnetic axes for the equigranular granites. Sample 146a shows a good correlation of the ferrimagnetic axes with the AMS ellipsoid while sample 136c only shows a weak correlation.

Fig. 7.13: l, f and p parameters of the different samples. In sample 146aII the para- and ferrimagnetic anisotropies are equally developed while in sample 136c the ferrimagnetic linear anisotropy is much better developed than the paramagnetic anisotropy.

7.6 Porphyritic granite

Seven samples were chosen for the separation using the HFA with bulk susceptibilities ranging between 175*10^-6 SI and 2055*10^-6 SI, the shape of the respective magnetic ellipsoids ranges between oblate and prolate, no neutral samples were separated. Most of the samples show a domination of the ferrimagnetic properties over the paramagnetic properties. From the U-values a constructive superposition of the para- and ferrimagnetic axes can be inferred for

0.01

sample 74c (see Fig. 7.14). The orientation of the axes points to an addition of properties since the AMS-axes are oriented as a geometric mean between the ferrimagnetic and paramagnetic axes (see Fig. 7.15). The other samples show all kinds of interaction (constructive or destructive) and the axes are rotated or interchanged (see Fig. 7.15).

Fig. 7.14: U-parameters of the different samples.

Most of the samples show a domination of the ferrimagnetic over the paramagnetic properties which is confirmed by the moderate to high bulk susceptibilities of the samples.

Fig. 7.15: Orientation of the main magnetic axes. Usually a good correlation between the ferrimagnetic ellipsoids and the AMS ellipsoids is found. Sample 74c shows an interaction of paramagnetic and ferrimagnetic axes leading to the measured AMS ellipsoid.

99a 96a 8a ^a

74c 46b 25b

24c 19b

k₂ k₃

Para

Ferri

AMS

N N

N N N N N N

k₁ k₂ k3

k₂ k3

-1 -0.5 0 0.5 1

99aI 96a 74c 46b 25bI 24c 8aBIII 19b

Para Ferri AMS

Fig. 7.16: l, f and p parameters of the samples.

From their l-, f- and p-ratios most of the samples show more pronounced ferrimagnetic fabrics than paramagnetic fabrics (see Fig. 7.16). Only samples 96a and 74c have dominating paramagnetic linear anisotropies, in sample 24c (p is exactly 1) both anisotropies from both contributing phases are equally developed.

7.7 Implications of HF-analyzes

The magnetic properties of the majority of the samples are controlled by ferrimagnetic minerals, sample 136c of the equigranular granites is controlled by another mineral (probably (anti)ferromagnetic) that can not be separated using the applied calculations. However, even if the bulk magnetic properties of the samples are controlled by the ferrimagnetic fraction of the sample the paramagnetic properties usually are parallel or subparallel to the ferrimagnetic properties. Some of the samples with high bulk susceptibilities show a destructive correlation between the respective main magnetic axes (paramagnetic axes are interchanged or rotated with respect to the ferrimagnetic axes). They are controlled solely by the ferrimagnetic minerals and their orientation of AMS-ellipsoid axes reflects the orientation of the main magnetic axes derived from ferrimagnetic minerals.

Nevertheless, the parameters calculated (U-parameter) for the ferrimagnetic and paramagnetic ellipsoids are comparable. The question arises how this difference between samples with high bulk susceptibilities may be explained. This phenomenon is not restricted to specific lithological units, grain sizes or ages which rejects the explanation through different origin or emplacement history of the units. Furthermore, the distribution of these samples is not restricted to certain areas in the batholith or in the different units in the batholith so no different tectonic modification of the samples can be assumed. Whether the different magnetic axes

0.01 0.1 1 10 100 1000

8aiii 19b 96a 99ai 46b 24c 74c 25bi

l f p dominating paramagentic

dominating ferrimagnetic

correlate constructively, destructively or are rotated with respect to other axes seems to depend on the distribution of ferrimagnetic minerals in the samples.

Where ferrimagnetic minerals are aligned in such a way that their magnetic properties may interact, a disturbed distribution of the ferrimagnetic axes is an explanation (Gregoire et al., 1995). The orientation of the magnetic ellipsoids obtained from AMS measurements depends on whether the samples are dominated by ferrimagnetic minerals (then the AMS-axes correlate with the ferrimagnetic ellipsoid) or whether they are controlled by the paramagnetic minerals in the sample (then the AMS-axes correlate with the paramagnetic ellipsoid). This is confirmed by the ratio of energy densities. Samples with a ratio

>1 (controlled by paramagnetic minerals) always show a subparallel distribution of magnetic axes while samples with interchanged or rotated magnetic axes all show ratios <1. The inverted conclusion that all samples with ratios of energy densities

<1 show rotated or interchanged axes does not hold true as it depends on the distribution of ferrimagnetic minerals in the samples.

Since the rotation of axes is restricted to samples with high bulk susceptibilities it may be a calculation error during the separation of the ferri- and paramagnetic properties. When only small amounts of paramagnetic minerals are present in the rock (e.g. in the porphyritic granite) that furthermore are nearly isotropic (k1~k2~k3) it may be possible that axes with comparable susceptibilities (e.g. in biotites with k1~k2>k3) are swapped. This would as well explain the observed exchange in orientations of paramagnetic axes with respect to the AMS measurements and the ferrimagnetic axes. This is confirmed by the observation that in samples where the shape of the paramagnetic ellipsoid is prolate, the paramagnetic k1-axis matches with k1 of the AMS measurements while k2 and k3 (paramagnetic) may be exchanged. In samples where the paramagnetic ellipsoid shows an oblate shape of the magnetic ellipsoid k3 (paramagnetic) and k3 (AMS) are subparallel to each other while k1 and k2 (paramagnetic) may be exchanged with respect to the AMS measurements. Finally, samples where the shape of the paramagnetic ellipsoid is neutral all three paramagnetic axes (k1, k2 and k3) may be exchanged or even rotated with respect to the AMS axes. According to this the orientation of the different paramagnetic axes are correlated with the respective AMS axes even if the magnetic rock properties are dominated by the ferrimagnetic properties. If only low amounts of paramagnetic minerals are present in the sample at least the axis

corresponding to the preferred shape of the magnetic ellipsoid (k1 if –1<U<0 or k3

if 0<U<1) are subparallel. It may be possible that in this case the exchanged axes (k2 and k3 or k1and k2 respectively) are subparallel to the corresponding AMS axes even if they are calculated otherwise.

However, from the l, f and p values of the samples it appears that in samples that show a domination of ferrimagnetic over paramagnetic properties, the degrees of linear and planar anisotropy as well as the magnitude of anisotropy are controlled by the ferrimagnetic minerals. Hence these values from the AMS measurements do not reflect the distinctness and degree of the magnetic fabric of the biotites and/or hornblendes and should therefore not be used for the interpretation of the paramagnetic mineral fabric.

101

8. Calculating theoretical bulk susceptibilities

Two different methods were used to calculate a theoretical bulk susceptibility of the samples. In one set of calculations the modal content of the samples was used while in the second set the FeO, Fe2O3 and MnO content was used.

8.1 Calculations using the modal content of the samples

Calculations of theoretical bulk susceptibilities can be performed using the modal content of the samples by simply adding the different mineral susceptibilities according to their percentage content (see equation [1], chapter 5.3). Since no ferrimagnetic minerals were calculated from the geochemical analyzes the calculated susceptibility only matches the measured susceptibility if no ferrimagnetic minerals are present in the sample. It has been shown by Siegesmund & Becker (2000) that in case of a difference between the calculated and measured susceptibility the difference must be assigned to the ferrimagnetic minerals. For samples with a reasonable hornblende content two different susceptibilities were used for the calculation (see Table 8.1 for mineral susceptibilities), one set with a highly and one with a less susceptible hornblende.

Table 8.1: Mineral susceptibilities and anisotropy values of the different main magnetic axes used for the calculations. Data for magnetite grains was calculated (Angenheister & Soffel, 1972) from metamorphic and granitic rocks, since the susceptibility of magnetite is shape preferred only the bulk susceptibility should be used. Data from (1) Borradaile et al. (1987); (2) Friedrich (1994); (3) Angenheister & Soffel (1972); (4) Hrouda (1986)

Ref. kbulk (*10^-6 SI) k1 k2 k3 P Remark s

Biotite 1 1180 1.098 1.095 0.832 1.32 kx <c>

ky <c>

kz || <c>

Hornblende (high hbl)

2 1306 1.037 1.023 0.94 1.1 kx || <b>

ky || <c>

kz || <a>

Hornblende (low hbl)

2 425 1.066 1.036 0.898 1.19 kx || <b>

ky || <c>

kz || <a>

Magnetite 3 ~600000 1.108 0.964 0.936 1.18 mean

Quartz 4 -13.4 isotropic

Plagioclase 1 -2.7 isotropic

K-feldspar 4 -12 isotropic

Results of the calculations are presented in Table 8.2. Calculations resulted for nearly all of the samples in bulk susceptibilities that are too low, an influence of

ferrimagnetic minerals must be assumed for most of the samples. The influence of ferrimagnetic minerals on the bulk susceptibility is confirmed by the marked difference in the actually measured values. While the arithmetic mean of the bulk susceptibilities of the different units was calculated to values well above 1000*10^-6 SI, the lowest measurement of samples is usually well below 100*10^-6 SI. If a specific magnetite content is added to the calculated values to match the arithmetic mean of actually measured values, the magnetite content of the different units is calculated to 0.27% for the southern gneissic granites, 0.33% for the northern gneissic granites and 0.18% for the western gneissic granites.

Calculations for the equigranular granites resulted in 0.24% of magnetite while the porphyritic granite should contain 0.45%. These amounts were calculated using a magnetite with a bulk susceptibility of 600000*10^-6 SI (see Table 8.1).

Table 8.2: Results of the calculations of the theoretical bulk susceptibility based on the average mineral content of the samples. Since more than one measurement core has been measured the

Im Dokument The emplacement of the Chinamora Batholith (Zimbabwe) inferred from field observations, magnetic- and microfabrics (Seite 99-0)